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Equipment Discussions >> Eyepieces

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BillP
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Eyepiece Performance using a Prism Diagonal
      #6224960 - 11/30/13 11:14 AM

I had the opportunity last evening for a first look evaluation of a Prism Diagonal vs. a Dialectric Mirror diagonal. The Prism was the Tak 1.25" (on loan) and the Dialectric was the AstroTech 1.25". Scope used was the TSA-102 and just two targets, Jupiter and M42. Eyepieces were the 6mm ZAO-II and the 5mm XO for planetary. For M42 Sterling Plossls were used, somtimes Barlowed.

Bottom line was that the Prism won the show on both targets, producing a definite notch better results on Jupiter and being subtly better on M42.

The Tak Prism provided a clearer view of a white spot transit, showing greater perceived contrast as well as crisper definition. Structure within the main belts similarly had greater perceived contrast and crisper definition, with more detail being visible as a result. The polar region differences were even more pronounced. In the Prism there was a lot of detailed structure visible instead of just being a simple gradient of shades. The Dialectric showed none of this. Scatter around Jupiter appeared at about the same extent in both diagonals, however it was not as bright in the Prism. Finally, and oddly IMO, the focus snap was much more pronounced using the Prism. Overall I felt like these two eyepieces were performing at what I would characterize as a level better in planetary performance, similar to what I might see going from a lesser performing eyepiece to the ZAOs.

Moving to M42 I was looking for three things: faintest star transmission, nebula extent, and mottled structure performance. With transmission, both diagonals were performing the same relative to stars. In both Trap-E was just as well defined (F was not showing this evening) and adverted vision stars were showing just as well. Extent of the nebula was slightly better in the Prism, with the outer wings of M42 showing well with adverted vision in the Prism and barely at all with the Dialectric. The brightness of M43 was also more obvious in the Prism vs the Dialectric. Finally, the mottled structure presented itself close to the same with each diagonal, but my impression was that it was just a little more detailed with the Prism. This only became somewhat apparent when the magnification was increased -- not apparent using the 17mm (48x) but then seemingly better with the 2.5x Barlowed 25mm (82x). So not a firm call here.

As a side note, I do have an old Celestron 1.25" Prism from the 1980's, but wanted to see how a quality modern Prism performs against a modern Dialectric. FWIW, a surprise for me was that the light path of the Tak Prism was fairly shorter than the old Celestron Prism. Something I was not expecting.


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russell23
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6224986 - 11/30/13 11:28 AM

I have a Tak 1.25" Prism and concluded much the same thing when I first got it. It definitely was an improvement upon the Williams optics diagonals (1.25" and 2" dielectric).

Currently I am using a 2" AP Maxbright diagonal, but I still have the Tak diagonal. The main thing I don't like about the Tak diagonal is that the nosepiece is not threaded for filters. Optically though it is great.

Dave


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PowellAstro
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Re: Eyepiece Performance using a Prism Diagonal new [Re: russell23]
      #6225013 - 11/30/13 11:42 AM

This all makes good since because the mirror will transmit 99 percent of off axis light and the prism will not. It will also add a small amount of SA correction.

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dbowlin
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Re: Eyepiece Performance using a Prism Diagonal new [Re: russell23]
      #6225023 - 11/30/13 11:46 AM

Bill, how did the 80's prism do against the TAK. Is the TAK worth the extra $$?
Dale


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SteveC
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Re: Eyepiece Performance using a Prism Diagonal new [Re: dbowlin]
      #6225073 - 11/30/13 12:15 PM

Where can I get prism conversion kits.

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george tatsis
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Re: Eyepiece Performance using a Prism Diagonal new [Re: SteveC]
      #6225312 - 11/30/13 02:28 PM

I wonder if anybody has ever compared an inexpensive 1.25" prism diagonal with a premium unit.

Hope Bill conducts more experiments , so that we get more info on it

George


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mgwhittle
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Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6225529 - 11/30/13 04:49 PM

Interesting observation Bill. I have a Zeiss diagonal that I use with a binoviewer but I have not used it by itself since I keep hearing that a prism will supposedly hurt the performance of a highly corrected APO. I think I have that right. Anyway, your TSA is an excellent APO so I might have to try the Zeiss out with my AP.

Thanks for posting.


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BillP
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Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6225574 - 11/30/13 05:16 PM

Quote:

I wonder if anybody has ever compared an inexpensive 1.25" prism diagonal with a premium unit.

Hope Bill conducts more experiments , so that we get more info on it

George




I will compare it to my Celestron, but the Celestron I have never b een satisfied with...and it has some problems as a small part of the FOV on one side vignettes from something internal or maybe the prism is not entirely square. I've had it apart but have never been able to fully resolve what the issue is going on. So not sure my 1980's prism is representative for a comparison. \\With the Tak, while I was not looking for any added color, I did not notice any with last nights observing.


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PJ Anway
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6225579 - 11/30/13 05:18 PM

Nice report Bill!

I find from experience that I prefer a prism over a mirror diagonal. I use an older Zeiss M44 prism diagonal in my f5.6 refractor and am very satisfied with the performance.


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Cyclop_si
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6225590 - 11/30/13 05:22 PM

Once I used low cost prism diagonal, and what I really did not like was "line spike" on bright stars at high magnifications (two spikes 180° apart). I thought this is due to prism diagonal design, but reading how good Tak prism performed, it looks like this might be due to poor prism build as well. You probably have not noticed anything like that spike on Tak diagonal, right?

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JustaBoy
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Cyclop_si]
      #6225608 - 11/30/13 05:36 PM

David,

That sounds like a 'correct image' prism star diagonal to me.

I have many cheap 1.25" prism diagonals from the 60s and 70s (not correct image) and see these spikes in none of them.

Thanks,

-Chuck


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BillB9430
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Re: Eyepiece Performance using a Prism Diagonal new [Re: JustaBoy]
      #6225928 - 11/30/13 09:03 PM

+1 on Chuck's reply, David. Amici prisms (90 degree) and Schmidt prisms (45 degree) both have a "roof" with an edge in the center of the field. This roof portion of the prism reverses right and left to give a fully corrected image. Diffraction of light hitting the "roofline" creates a horizontal spike visible both directions from every bright object. Better quality correct image prisms have a finer edge and so a fainter spike, but all have it. Regular right angle prism astronomical diagonals show an erect image but it is reversed right and left - no roof in these, hence there should be no spike. - Bill

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urassner
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6226031 - 11/30/13 10:13 PM

Bill, in some older CN threads it was mentioned that Prism diagonals are not suitable for shorter focal length telescopes (f10 and faster) due to refraction effects when light enters at an angle creating chromatic aberration. I noticed that you used a f8 refractor and it did not seem to be an issue. Might it be more of an issue with 2 inch diagonals and lower magnification? I would appreciate your insight.

Thank you, Ulrich


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JustaBoy
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Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6226037 - 11/30/13 10:19 PM

I think the critical ratio is more like F/6, or there a bouts.

I use one on my Orion F/5 ST-80, but who could tell the difference:-)

-Chuck


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urassner
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Re: Eyepiece Performance using a Prism Diagonal new [Re: JustaBoy]
      #6226074 - 11/30/13 10:36 PM

Thanks, Chuck, for your input. Were you using 1.25 inch or 2 inch eyepieces? Does it make a difference?

Ulrich


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JustaBoy
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Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6226099 - 11/30/13 10:49 PM

The ST-80 doesn't have a 2" focuser, so 1.25" Prism diagonal.

I don't really know if the size of the eyepiece field stop would make a difference or not.

Sorry,

-Chuck


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jrbarnett
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6226127 - 11/30/13 11:07 PM

I think the Tak prism uses different glass than most other prism diagonals. If so, I wonder if these results are unique to the Tak prism or rather would also apply to other prisms using different glass?

I have the Tak prism as well as both vintage and recent Japanese prism diagonals. I've never checked them head to head, but I have tested every diagonal I own and many I no longer own for scatter using calipers and a GLP (measuring contact spot diameters). Prism diagonals tested in this manner show very low scatter - better than the best dielectrics and on par with the best standard coated mirrors, which tend to be smoother than the dielectrics.

Regards,

Jim


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george tatsis
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Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6226220 - 12/01/13 12:28 AM

Quote:

Bill, in some older CN threads it was mentioned that Prism diagonals are not suitable for shorter focal length telescopes (f10 and faster) due to refraction effects when light enters at an angle creating chromatic aberration. I noticed that you used a f8 refractor and it did not seem to be an issue. Might it be more of an issue with 2 inch diagonals and lower magnification? I would appreciate your insight.

Thank you, Ulrich




Same question here!

I remember reading the previous claim in the past, and it was the reason for my not buying a prism diagonal.

However, I suspect Jim is right by pointing out that it may have to do with the Tak design / material used in this particular prism, and not all prisms in general.

It's definitely an issue worth looking into, because according to Bill's findings, for the price of a good eyepiece you can enhance your planetary viewing by simply upgrading to this particular diagonal.

I think that this is the least expensive solution to increasing planetary contrast, because all eyepieces used with this particular diagonal will be benefited by it.

It's even more than " kill two birds with one stone " kinda thing ,since on average, we have our three to four favorite planetary eyepieces taking advantage of these wonderfully unexpected diagonal properties!

George


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george tatsis
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Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6226226 - 12/01/13 12:35 AM

Just wondering how many CNers have been Googling "1.25 prism diagonal reviews" since Bill started this subject.

I know I have.

George

Edited by george tatsis (12/01/13 12:37 AM)


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urassner
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Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6226232 - 12/01/13 12:42 AM

I do not have experience with either diagonal. Could the observed difference be just the higher optical quality of the Tak diagonal compared to the Astrotech diagonal. Would you still see an improvement if it was compared to an AP, TV or Baader dielectric diagonal? I would assume that a Zeiss prism diagonal should be of similarly high quality as a Tak Diagonal and would also show a similar improvement as a Tak.

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george tatsis
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Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6226278 - 12/01/13 01:30 AM

Quote:

I do not have experience with either diagonal. Could the observed difference be just the higher optical quality of the Tak diagonal compared to the Astrotech diagonal. Would you still see an improvement if it was compared to an AP, TV or Baader dielectric diagonal? I would assume that a Zeiss prism diagonal should be of similarly high quality as a Tak Diagonal and would also show a similar improvement as a Tak.




PJ's reference to his excellent Zeiss prism diagonal corroborates Bill's report.

It's only logical to assume that quality and execution of design are of the utmost importance, because both samples come from reputable companies (Tak,Zeiss).

George


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Roy McCoy
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Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6226287 - 12/01/13 01:40 AM

I now use a Baader T2 with the Zeiss prism in an Orion 80 ED f7.5 and a 2” Zeiss prism in an 86’ AP 6" f8.

Incidentally, I feel the T2 Baader Zeiss prism outperforms the 2” Zeiss prism. Maybe the shorter path? But I use 2 inch ep’s in the 6” so I’ll keep it.

As Bill said “a definite notch better”. There were several of us testing the diagonals and all involved agreed the difference was not subtle. To me the difference is as pronounced as the difference between an achro and apo.

Dust lanes and mottling pop so much more with the prism in my longer focal length scopes.

I found that the Baader 2”mirror is the better performer in the AT106 f6.5.


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urassner
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Roy McCoy]
      #6226302 - 12/01/13 01:50 AM

Thank you for the information. I am using an older AP refractor as well. I should think about a prism diagonal.

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Ernest_SPB
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Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6226331 - 12/01/13 02:36 AM

I have 2" Amichi prizm from baader.

It is nice to see objects in right orientation. But so thick piece of glass introduces noticeable chromatic aberrations into image of my 80 mm F7.5 ED-apochromat. Then roof edge makes some artifacts: two strong rays into opposite sides of bright objects (like Jupiter, Venus) and distort a bit fine features in high magnification image (like astigmatism or doubling).

I think 2" prisms should not be used if you want to get image with high resolution/contrast.


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ivan86
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Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6226424 - 12/01/13 05:26 AM

I have a Carl Zeiss 2" prism diagonal and i have used it with a Skywatcher 100 ED and now with my new Meade ACF 12 " SC.

This diagonal when compared to standard dieletric mirror 99% diagonal such WO / TS show better definition on planets and better contrast agaist background on deepsky objects.

The difference is more noticeable when i look at low contrast structure inside nebula.

Ill use in the future only prism diagonal and ill keep my Carl Zeiss (350 euro) forever.

Ivan


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Heavens Above
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Re: Eyepiece Performance using a Prism Diagonal new [Re: ivan86]
      #6226633 - 12/01/13 10:11 AM

I compared a Baader Zeiss prism diagonal (1.25) against a Astro Physics Maxbright, which must be the best mirror diagonal made and decided to keep the prism. On planets I found it had less scatter and greater clarity. These results were the same on an TMB 115 Apo and a Tak Mewlon 210.

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Sarkikos
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillB9430]
      #6226696 - 12/01/13 10:40 AM

Quote:

+1 on Chuck's reply, David. Amici prisms (90 degree) and Schmidt prisms (45 degree) both have a "roof" with an edge in the center of the field. This roof portion of the prism reverses right and left to give a fully corrected image. Diffraction of light hitting the "roofline" creates a horizontal spike visible both directions from every bright object. Better quality correct image prisms have a finer edge and so a fainter spike, but all have it. Regular right angle prism astronomical diagonals show an erect image but it is reversed right and left - no roof in these, hence there should be no spike. - Bill




IME, Amici prisms are fine for low power use. I have one in my 70mm finder scope on my 10" Dob. I prefer the natural orientation when star hopping. (Why wouldn't I?) But for higher magnification, the diagonal or prism which gives a reversed image also gives a sharper image.

Mike


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Philip Levine
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6226698 - 12/01/13 10:41 AM

thanks Bill,
Very informative post. I'm always looking for ways to improve visual observing detail, the prism diagonal info is most welcome.
Phil


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Sarkikos
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Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6226705 - 12/01/13 10:44 AM

Quote:

Just wondering how many CNers have been Googling "1.25 prism diagonal reviews" since Bill started this subject.

I know I have.

George




I still have many of these threads in my favorites here on CN.


Mike


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ibase
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6226715 - 12/01/13 10:49 AM

If one is going prism on the diagonal, don't cheap out - got a low-cost prism diagonal and its performance is less than desirable compared to the mirror diagonals.

Best,


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Sarkikos
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6226716 - 12/01/13 10:50 AM

If you're going to use the prism diagonal mostly for moderate to higher power work, would it be worthwhile to buy a 2"? Why not go with a 1.25" prism diagonal?

A 2" dielectric mirror diagonal should be good enough for low-power wide-field. And a 1.25" Amici would be best for a natural orientation finder scope.

Mike


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BillP
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Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6226721 - 12/01/13 10:51 AM

Quote:

Bill, in some older CN threads it was mentioned that Prism diagonals are not suitable for shorter focal length telescopes (f10 and faster) due to refraction effects when light enters at an angle creating chromatic aberration. I noticed that you used a f8 refractor and it did not seem to be an issue. Might it be more of an issue with 2 inch diagonals and lower magnification? I would appreciate your insight.

Thank you, Ulrich




Yes. Tammy mentioned this to me as well. I do have an 80mm f/6.25 APo so will plan to give the 2 diagonals a compare on that scope as well. I will also do some low magnification testimng as well on bright objects.

btw, has anyone compiled a list of current production prism diagonals (not the amici types)? I was Googling for a full 2" one that would accomodate a full 46mm clear light path so could use max TFOV 2" EPs. Came up with nothing so far.

Edited by BillP (12/01/13 10:58 AM)


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Sarkikos
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6226738 - 12/01/13 10:56 AM

Here's an old CN thread on prism diagonals:

Prism vs. Mirror

Mike


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Sarkikos
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6226744 - 12/01/13 10:58 AM

Here's another:

Prism Diagonal

Mike


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Sarkikos
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6226763 - 12/01/13 11:10 AM

If I upgrade to a better prism diagonal I will use it mostly in my f/10+ SCT's and Maks for moderate to high magnification. IMO, it would make more sense to get a 1.25" diagonal.

Which is the best 1.25" prism diagonal? Which ones are almost as good?

Mike


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Roy McCoy
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6226786 - 12/01/13 11:24 AM

Hi Bill,

AlpineAstro has the 2" Zeiss, it is listed on the price page as PRISM-2.


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urassner
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6226799 - 12/01/13 11:29 AM

Bill, I spent some time yesterday after I read your report and could only find one 2 inch version: The Baader 2 inch with a Zeiss prism, available directly from Baader or from Agena astro.

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george tatsis
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6226809 - 12/01/13 11:33 AM

Quote:

If I upgrade to a better prism diagonal I will use it mostly in my f/10+ SCT's and Maks for moderate to high magnification. IMO, it would make more sense to get a 1.25" diagonal.

Which is the best 1.25" prism diagonal? Which ones are almost as good?

Mike




Apparently not many 1.25" options are currently available.

The Tak Bill tested is supposed to be the best one on the market.

https://www.astronomics.com/takahashi-1-25-inch-prism-diagonal_p3349.aspx

Baader T-2 is another option of course!

http://www.alpineastro.com/Star_Diagonals/Star_Diagonals.htm

Last but not least, we have the Vixen!

http://www.astroshop.eu/vixen-diagonal-prism-90d-1-25-/p,5833

TS

http://www.teleskop-express.de/shop/product_info.php/info/p604_T2-diagonal-pr...

El cheapos are not included here!

Take a pick Mike!

George

Edited by george tatsis (12/01/13 11:51 AM)


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Hesiod
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Roy McCoy]
      #6226833 - 12/01/13 11:39 AM

I have the Baader-Zeiss 1.25" and I feel that it is better than any other prism I have ever tried, but the "standard" Baader prism follows him quite closely.
Anyway I have not ever tried the Takahashi (which at 90€ could be a real bargain) and I use the prism mainly behind my 8" SCT which somebody could judge a rather junkish telescope compared to a TSA.


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Sarkikos
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Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6226837 - 12/01/13 11:41 AM

I'm looking at the T-2 Prism Star 1.25" Diagonal. That is supposed to incorporate a Zeiss prism with Baader coatings. How does this prism diagonal compare with others?

Mike


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george tatsis
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Hesiod]
      #6226840 - 12/01/13 11:42 AM

Quote:

I have the Baader-Zeiss 1.25" and I feel that it is better than any other prism I have ever tried, but the "standard" Baader prism follows him quite closely.
Anyway I have not ever tried the Takahashi (which at 90€ could be a real bargain) and I use the prism mainly behind my 8" SCT which somebody could judge a rather junkish telescope compared to a TSA.




Where did you find the Tak for 90 Euros?

George


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Sarkikos
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6226843 - 12/01/13 11:43 AM

Hesiod,

You anticipated and answered my question.

Mike


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Sarkikos
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6226852 - 12/01/13 11:46 AM

Also, check this out:

Improving Refractor Performance with a Prism Diagonal

Mike


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Hesiod
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6226857 - 12/01/13 11:49 AM

I do not know if it is the same BillP is speaking of, but the EU official importer optique unterlinden has a 1.25" prism at 89€ (and a mirror at 400€...), sub voce "Renvois coudés"

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Sarkikos
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6226858 - 12/01/13 11:49 AM

What is this? It doesn't say it has a Zeiss prism, but ...?

Baader T-2 90° Prism Star Diagonal Body # T2-01C 2456005

Mike


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george tatsis
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6226870 - 12/01/13 11:56 AM

Quote:

What is this? It doesn't say it has a Zeiss prism, but ...?

Baader T-2 90° Prism Star Diagonal Body # T2-01C 2456005

Mike




Neither the nosepiece, nor the eyepiece holder is included!!!

Take a look at the TS option I previously mentioned. It covers both!

George

Edited by george tatsis (12/01/13 11:59 AM)


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Eddgie
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Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6226879 - 12/01/13 12:01 PM

This version comes with 1.25" nose and micro-focusing eyepiece holder for $159.

Baader 1.25" diagonal with nose and eyepecie holder


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Hesiod
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6226889 - 12/01/13 12:05 PM

Quote:

Hesiod,

You anticipated and answered my question.

Mike


You are too fast, or I am too slow

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Eddgie
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6226908 - 12/01/13 12:13 PM

From the review you posted...

Quote:

Owners of refractors by Astro-Physics, Takahashi, TeleVue, TMB and other very premium makes likely won't see significant performance benefits from using a prism.




I have a SV110ED and a 6" f/8 refractor.

I have the Baader Prism (non Zeiss) and the Maxbright Mirror Dielectric.

I have also owned the Tak several years ago and did A/B with the AP with both the Tak and a Televue Dielectric.

Sady, my result was not the same as the OPS.

In the 110ED, I actually felt that the Maxbright Mirror gave a tiny difference in performance, but it was not in any way meanigful, but I like the longer light path of the mirror, so I continue to use it.

In the 6" APO though, nada. Both the Tak prism and the Baader (non-zeiss) gave as best as I could determine, identical views. I could see absolutly no difference in detail between the two units.

Now a change that did matter was going to a GPC in the 110ED with binoviewers for solar white light, and going from a glass filter to a Baader Film filter.

In these cases, the differecne in performance was quite noticable.

But the difference between the prism and the mirror, at least at f/8 and an Astro-Physics APO was to small a difference for me to see.

So to me, this report from the OP is clearly in the "Your mileage may vary" catagory.

This comes around from time to time, and people will say that the prism made a big difference, and others will say that it did not.

Like so many dialogs on CN, there are arguments on both sides of any proclamation (this is better than that).



Most of the times I have tired the "Miracle Cure", I have been disappointed.


The GPC in the binoviewer and the Baader Film filters were two excepetions.

The "dielectric diagonal" mania was a huge disappointment. The dielectric performed no better than the Televue Enhanced Aluminum.

The XLT coatings was another, with initial hysteria reports of big performacne increases, when it is a struggle to see the difference between XLT and Starbright visually.

So, not saying that people should not try a Prism. The Baader standard is a great prism to be sure. I own 2 and use them with my binoviewer (for the short light path).

But for uses where I don't need a short light path, tried the prisms, went back to the mirror. Not that it was better optically, but it is a heck of a lot easier to clean than a Prism.

Edited by Eddgie (12/01/13 12:17 PM)


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Hesiod
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Hesiod]
      #6226915 - 12/01/13 12:16 PM

The Baader prism are a sort of "LEGO", you may choose the nose (even a 2" if you will) and the eyepiece holder, either from Baader or from any other (the prisms are T2
threaded, male on the EP side and female on the scope side).
I do not recommend the non Click-Lock holder because it
is a real hassle change eyepiece, to the point that often I use instead a 3cm extension tube whit a brass compressing ring


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Sarkikos
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Eddgie]
      #6226976 - 12/01/13 12:41 PM

Quote:

This comes around from time to time, and people will say that the prism made a big difference, and others will say that it did not.

Like so many dialogs on CN, there are arguments on both sides of any proclamation (this is better than that).




Yes, I've read many of these threads. Much heat, little light. Well, not really so much heat ... but really little light, that I could see. Basically it distills to: "Choose your experts and purchase accordingly."

But once again I think I'll pass on an upgrade. A high-end prism diagonal is still on my list, but not my short list. We'll see...

Mike


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jrbarnett
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Eddgie]
      #6227126 - 12/01/13 01:49 PM

However, there is no question that prism diagonals introduce slight overcorrection and mirror diagonals do not, so assuming a refractor is not under-corrected, a prism introduces spherical aberration whereas a mirror does not. You can also see from many, many reports on astroforen.de that introduction of a prism diagonal in many different refractor systems alters color correction; generally to the better. See Rohr's description of "RC Index" and note how that metric alters when a "glasweg" (prism diagonal) is introduced. For example:

http://www.astro-foren.de/showthread.php?10380-Zeiss-APQ-100-640-Glasweg

So if your scope was perfectly corrected for spherical, the prism should have produced a worse quality image to some degree. Note, though, that the color shift properties of the prism are not limited to under-corrected scopes. That APQ tested in the link was pretty well figured prior to introduction of the prism.

I think rational diagonal choice requires that the user know something about both the effects of a prism on the system and the state of correction of his refractor. Otherwise it's all pretty much seat of the pants guesswork.

Regards,

Jim


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Roy McCoy
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Re: Eyepiece Performance using a Prism Diagonal new [Re: jrbarnett]
      #6227235 - 12/01/13 02:32 PM

Quote:

So if your scope was perfectly corrected for spherical, the prism should have produced a worse quality image to some degree.




My AT106 f6.5 did get worse with the prism.



Quote:

I think rational diagonal choice requires that the user know something about both the effects of a prism on the system and the state of correction of his refractor.




I agree.


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BillP
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UPDATE: Prism Diagonal new [Re: urassner]
      #6227348 - 12/01/13 03:39 PM

Guys,

Have an update. A fellow CNer on PM asked me to check the collimation of the 2 prism diagonals. I did not do this prior to the 1st light as just assumed the Tak would be ok.

To do the test I took my 80mm APO and removed the objective so I would have a mounted platform to take the diagonals. I then projected the laser dot on a wall about 20 feet away with no diagonal in the focuser. I then marked the spot as the reference spot. I also rotated and reinserted the laser several times to ensure a consistent location. It was. I then put the AstroTech Dialectric in the scope housing and reshot the laser. It landed on the same spot so this diagonal was fully square with the focuser. When I put the Tak Prism in the scope housing and attached the laser and tightened the Tak's collar on the laser, the spot painted the wall about 2" low!! What a surprise...it was off and not square yet it still gave a better view. I also noted that if the eyepiece is not tightened in the Tak Prism then it is way further off than the 2". So fully tightening the Tak collar is important. The conventional compression ring of the AT Dialectric was not as sensitive when the ocular was not tightened in it.

I have added a few layers of painters tape to the Tak Prism's housing so when it sits in the focuser it now paints the laser dot accurately. Will redo the experiment later this week when things clear up. So...should I expect it to perform even better now? I mean it beat the Dialectric uncollimated...can't wait to see what occurs now.

PS - I will also conduct tests with a short f/6 refractor as well to see how they compare with shorter focal ratio...and will try some low & high magnification bright star observing to see if that teases out any color.


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junomike
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Roy McCoy]
      #6227475 - 12/01/13 04:50 PM

Quote:

My AT106 f6.5 did get worse with the prism.




Same thing for me with my AT111EDT....Twice!
The first Tak 1.25" Prism induced CA that was not otherwise seen. It was soon sold. Then due to these type of threads I figured It might have been a dud. A second one was purchased. Result..........same! The Tak Prism induced CA on a friends Meade 102ED as well.

Hey Jim, give It a go with you AT111EDT and see what result you get!.

Mike


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BillP
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Re: Eyepiece Performance using a Prism Diagonal new [Re: junomike]
      #6227564 - 12/01/13 05:39 PM

Mike,

What was the magnification and targets where the CA showed up in your testing? Want to see if I can replicate or come close to same conditions.


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skullpin
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6227692 - 12/01/13 06:35 PM

I used an AP MaxBright, a Tak 1.25 prism, and the Zeiss 2" prism with my f/6 Borg 125SD scope, my findings on Jupiter last winter at 200-250X:

AP MaxBright: At optimum focus, color fringing appeared well stamped out to my eye. Greenish on one side of focus, purple on the other, fairly easy to achieve optimum focus with no fringing visible.

Tak prism: Fringing was a little more finicky, though it was possible to achieve an optimum focus with no color fringing. I was convinced that this prism gave a more detailed view than the dielectric mirror.

Zeiss 2" prism: Fringing was quite finicky, and it was almost impossible to achieve an optimum color free focus. The detail visible was no different than the dielectric mirror.

I sold the Zeiss prism, and, regrettably, the Tak prism too. I am tempted to find another Tak or perhaps the Baader T2 Zeiss prism.

Keith


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Sarkikos
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Re: Eyepiece Performance using a Prism Diagonal new [Re: skullpin]
      #6227821 - 12/01/13 07:52 PM

OK, I hear a lot of talk about 'fractors. I mostly use them for finders on my other scopes. Just sayin'.

Now how will a good prism diagonal perform vs a mirror diagonal in an SCT or Mak? I supposed that will depend on whether the Cat is undercorrected or overcorrected or has a good figure?

Mike


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fjs
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Re: UPDATE: Prism Diagonal new [Re: BillP]
      #6227826 - 12/01/13 07:53 PM

Quote:

Guys,

Have an update. A fellow CNer on PM asked me to check the collimation of the 2 prism diagonals. I did not do this prior to the 1st light as just assumed the Tak would be ok.

To do the test I took my 80mm APO and removed the objective so I would have a mounted platform to take the diagonals. I then projected the laser dot on a wall about 20 feet away with no diagonal in the focuser. I then marked the spot as the reference spot. I also rotated and reinserted the laser several times to ensure a consistent location. It was. I then put the AstroTech Dialectric in the scope housing and reshot the laser. It landed on the same spot so this diagonal was fully square with the focuser. When I put the Tak Prism in the scope housing and attached the laser and tightened the Tak's collar on the laser, the spot painted the wall about 2" low!! What a surprise...it was off and not square yet it still gave a better view. I also noted that if the eyepiece is not tightened in the Tak Prism then it is way further off than the 2". So fully tightening the Tak collar is important. The conventional compression ring of the AT Dialectric was not as sensitive when the ocular was not tightened in it.

I have added a few layers of painters tape to the Tak Prism's housing so when it sits in the focuser it now paints the laser dot accurately. Will redo the experiment later this week when things clear up. So...should I expect it to perform even better now? I mean it beat the Dialectric uncollimated...can't wait to see what occurs now.

PS - I will also conduct tests with a short f/6 refractor as well to see how they compare with shorter focal ratio...and will try some low & high magnification bright star observing to see if that teases out any color.




Thanks Bill!


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junomike
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6227959 - 12/01/13 09:04 PM

Bill, The scope was the AT111EDT (FPL-51 F7 Triplet) and the target was the edge a 3/4 Full Moon.
I don't recall exactly, but I believe I was using a 5mm XO yielding 155X.

I didn't even get around to comparing other characteristics (scatter. etc) as the induced CA was something I couldn't get past.

Interesting though that Roy seen a worse Image as well with his AT106 although his is a little faster but uses FPL-53.

Mike


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BillP
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Re: Eyepiece Performance using a Prism Diagonal new [Re: junomike]
      #6228016 - 12/01/13 09:36 PM

And the fringing on the lunar limb was when it was positioned dead center of the FOV or not. If not, then can also be lateral color from the eyepice...although an XO would not have that. But if only lateral color, then that has no impact for an on-axis planetary view.

Anyway, thanks for the details. I will be sure to run some lateral color compares also as maybe the prism induces this. I did no lateral checks as was just doing on-axis planetary observing.

Edited by BillP (12/01/13 09:37 PM)


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Tank
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6228119 - 12/01/13 10:48 PM

Bill
I will back Mikes result from what i recall the prism showed me CA that was not in the 99% mirror diagonals
I felt that there a significant difference and liked the mirror diagonals for a few reasons.
In the APO triplets/ mirrored scopes i felt the prism made them a doublet scope which had a small amount of CA.
The Diagonals that were tested were the AP,WO 99%, Denk 99% from my results i would recommend a mirror over a prism.


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Tank
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6228120 - 12/01/13 10:48 PM

Bill
I will back Mikes result from what i recall the prism showed me CA that was not in the 99% mirror diagonals
I felt that there a significant difference and liked the mirror diagonals for a few reasons.
In the APO triplets/ mirrored scopes i felt the prism made them a doublet scope which had a small amount of CA.
The Diagonals that were tested were the AP,WO 99%, Denk 99% from my results i would recommend a mirror over a prism.


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jrbarnett
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Sarkikos]
      #6228276 - 12/02/13 12:56 AM

"Now how will a good prism diagonal work vs a mirror diagonal in an SCT or Mak?"

Similarly, it will depend on the scope, but you have so many other things going on with a Cat that it may be harder to get the scope in optimal condition to be able to isolate the rather subtle diagonal differences.

Regards,

Jim


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Fomalhaut
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6228387 - 12/02/13 04:29 AM

Bill,

a) When used without diagonal OR with the Baader Maxbright (140 layers of dielectric coatings) my Tak FCT100 f/6.4 doesn't show any false colour neither inside nor outside of focus. And what might be even more comprehensive, in these two configurations it shows its most perfect startest i.e. absolutely indistinguishable identity of defocused airy disks on either side of focus.

b) The Tak 44mm-prism diagonal introduces substantial overcorrection resulting in visible spherochromatism, less than perfect star-test and slightly reduced planetary performance. Main reason for all of this => presumably the telescopes' short f/6.4 focal ratio.

c) Compared to b), the smaller Tak 1.25-inch prism diagonal just shows a tiny little bit of spherochromatism as well as clearly better low-contrast Jovian surface detail. Compared to a), it shows only slightly reduced perfection of star-test (quite better than b: the 44mm-prism).
Surprisingly, the 1.25-inch Tak prism diagonal tends to show slightly better (!) Jovian surface detail even as when compared to either of the two seemingly perfect a)-configurations. Since there seems to be no difference in stray-light between a) and c), I do not have an explanation for this last comparison's diverging results...

Chris


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Sarkikos
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6228481 - 12/02/13 06:58 AM

Quote:

c) Compared to b), the smaller Tak 1.25-inch prism diagonal just shows a tiny little bit of spherochromatism as well as clearly better low-contrast Jovian surface detail. Compared to a), it shows only slightly reduced perfection of star-test (quite better than b: the 44mm-prism).
Surprisingly, the 1.25-inch Tak prism diagonal tends to show slightly better (!) Jovian surface detail even as when compared to either of the two seemingly perfect a)-configurations. Since there seems to be no difference in stray-light between a) and c), I do not have an explanation for this last comparison's diverging results...

Chris




Maybe the tiny bit of induced spherochromatism from the Tak 1.25 prism diagonal enhances contrast for Jovian surface detail in the same way that a good contrast filter works, by subtly shifting the color balance in the image? If so, it'd probably be better to just use a Baader Moon & Sky Glow, and keep the mirror diagonal in the focuser.

Mike


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BillP
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6228595 - 12/02/13 08:48 AM

Quote:

Surprisingly, the 1.25-inch Tak prism diagonal tends to show slightly better (!) Jovian surface detail even as when compared to either of the two seemingly perfect a)-configurations. Since there seems to be no difference in stray-light between a) and c), I do not have an explanation for this last comparison's diverging results...




Chris,

Perhaps a possibility is what Jim B. suggested, that if we know nothing of the main optic then really difficult if not impossible to figure out root cause. With spherochromatism in particular, most designs have some, as does the prism, so question becomes what is the exact spherochromatism plot for each wavelength of light in both. Knowing that is the only way to know how the prism may be impacting the overall image. So this then necessarily means it will vary by scope as well, and target we are observing. Jupiter is a fairly neutral colored object with more colors in the blue range than anything else. So performance on Jupiter may differ when compared to performance on Saturn, vs Mars, vs Luna. All depends on the the predominent colors involved with each target and how the prims's offset if compared to the main objective's offset with color performance.

While the root causes are indeed interesting, for me this is a system test by target. So how do these diagonals offer up the view of Jupiter differently in a TSA-102. As scopes are changed and targets are changed, I imagine results will change also. If we want to get down to the science of it so we can have a predictive model, that is well beyond anyone here as we would first need the bench tested results for each component relative to its spherochromatism and other factors.

I will definitely test the impacts by focal ratio as you pointed out as much as I can. Will be interesting to take my f/6 APO and run a test, then mask the objective to make it a longer focal ratio and repeat to see how things may have changed. Better to use same scope IMO than two different ones since that just adds more unknowns.

Thanks also for the comment on false color when just out of focus...another test I did not do


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jjack's
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6229156 - 12/02/13 01:31 PM

You must know that the speed of light is reduced when traveling into theese thick pieces of glass !
after a lot of time transiting into space, photons could be annoyed if they are slowed down just before there destination


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jjack's
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Re: Eyepiece Performance using a Prism Diagonal new [Re: jjack's]
      #6229168 - 12/02/13 01:39 PM

Seriously : my SC5 react the same way as an apo refractor. I can see some red or blue fringes around the external circle when i defocus slightly a bright star with a Mirror diagonal. This effect is reduced with a 1"1/4 prism diagonal, and disapear totaly with a 1"1/4 45° amici prism.
Images does not seem affected, but i keep the 90° prism diag.


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JustaBoy
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Re: Eyepiece Performance using a Prism Diagonal new [Re: jjack's]
      #6229176 - 12/02/13 01:43 PM

What's that you've been drinking

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junomike
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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6229298 - 12/02/13 02:36 PM

Quote:

And the fringing on the lunar limb was when it was positioned dead center of the FOV or not.




Bill, I placed the Lunar Edge in the vertical middle of the eyepiece and then slewed the scope so that the middle of the arc (Lunar edge was smack dab in the middle. It didn't seem to matter as the induced CA was visible all over the FOV. I can't explain It, but It's the only time I've ever seen CA in that scope aside from some very miniscule CA on Sirius @ 300X + (which most people wouldn't probably notice).

I was so disappointed (for the second time) that I had Tony offer an opinion. He was equally surprised.

Mike


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george tatsis
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Re: Eyepiece Performance using a Prism Diagonal new [Re: junomike]
      #6229405 - 12/02/13 03:14 PM

Just wonder if it's worth buying a 1.25" prism diagonal for my SW 120ED

Has anybody ever tried this set up?

George


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BillP
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Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6229534 - 12/02/13 04:04 PM

Quote:

Just wonder if it's worth buying a 1.25" prism diagonal for my SW 120ED

Has anybody ever tried this set up?

George




Well if anyone reading this chimes in that they have a SW120ED and they live close to Wanshington, D.C. (i.e., less than 1 hr away), then let me know and might take a trip out to them with the diagonals to experiment on that scope

Edited by BillP (12/02/13 04:05 PM)


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John Harold
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Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6230285 - 12/02/13 09:21 PM

I use an old Celestron 1.25" prism in my 120, but I'm not qualified to say whether it has an advantage over the dielectric that came with it: I've never attached it to the scope! I do this for weight reasons. That scope is my dedicated backyard planetary cruiser, and the 2" diagonal and adapter would make it that much heavier. I will say that in good seeing I can jack the power to 225 (4mm ortho)no problem, and I'm pleased to say that the detail is just as good or better than what I've seen in comparable scopes. Looking at stars I see no CA. I use a 2" prism in my 127 at star parties. When I bought it the consensus was that prisms pass more light, however that was before dielectrics came on the scene.

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Roy McCoy
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Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6230461 - 12/02/13 10:34 PM

Quote:

Just wonder if it's worth buying a 1.25" prism diagonal for my SW 120ED

Has anybody ever tried this set up?

George




Yes I have with a friend who was borrowing a SW120ED to evaluate.

We noticed that the color shift was not as pronounced as with the Orion 80ED, there was less fringing either side of focus. Mainly we saw a noticeable change in contrast and detail in nebula as has been mentioned several times in this thread. My astro buddy said that if he bought this scope, his very next purchase was going to be the T2 Prism diagonal. We tested it with the 2” prism and the contrast and color fringing was better than the mirror that came with the SW 120, but the color fringing was worse than with the T2 with contrast being about the same.

There were five of us comparing the diagonals that night. Each person easily noticed the difference straight away.

Unfortunately sky conditions deteriorated before we were able to put in a known good mirror diagonal to compare directly with the supplied mirror. So this is the only unknown unfortunately because perhaps the included SW diagonal is not up to comparison.

Off the topic of the SW120, in the Orion 80 ED 7.5, color fringing almost completely disappears in or out of focus with the T2 Prism. And of course the details are at another level. This was compared on different nights as from the SW. This comparison was against a AstroTech Quartz dialectric 2” diagonal.


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george tatsis
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Roy McCoy]
      #6230537 - 12/02/13 11:26 PM

John, Roy,

Your reports have been very informative! Thank you guys.

What I need now is a good Samaritan to email Bill to conduct his experiments for the final verdict!

George

Edited by george tatsis (12/02/13 11:31 PM)


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Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6230651 - 12/03/13 01:38 AM

I did a similar comparison about a year back (or maybe two?), compared my Tak 1.25" prism with my Baader ClickLock 2" diagonal (or it might have been the T2 Maxbright mirror, I don't remember) and with the stock 1.25" prism that came with the 8" EdgeHD. I compared them in the EdgeHD only, on stars only (no planets or moon as far as I can remember). The differences were subtle but repeatable:

1) The mirror showed no detectable color in focus.
2) The Tak did show some color around the stars.
3) The Celestron showed even more color than the Tak diagonal.

I don't remember about differences in scatter. A detail, my Tak prism diagonal has a fine hairline scratch off center, about 5-7mm long, I don't think it has any impact but just to let you know about that.

I was a little disappointed with the Tak so have not used it much since.

I've been meaning to redo the comparison on my 80mm apo and the 120mm Equinox if I ever get a clear night around here, George I'll let you know about the results with the Equinox when I've had the chance to try it out. I can see very little color in the Equinox (much much less than I was expecting) so it will be interesting to see if the prism makes it even better, or worse, in that regard.

One more thing, Bills last point about different results for different parts of the color spectrum I think may be the key to interpreting the different results some report here. I will try to compare Jupiter as well as stars the next time.


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Re: Eyepiece Performance using a Prism Diagonal new [Re: Ava]
      #6230665 - 12/03/13 01:52 AM

Hi Anders,

Try something in the 50X-70X on M42 with the Equinox.
And as low power as you can go in your diagonal on M31.

Regards


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Re: Eyepiece Performance using a Prism Diagonal new [Re: Ava]
      #6230784 - 12/03/13 05:02 AM

Thanks Anders,

Your results with the Equinox + the 1.25" Tak, and hopefully Bill's shoot-out will determine whether or not I'll go for it.

I keep my fingers crossed for some good results!

George


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george tatsis
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Re: Eyepiece Performance using a Prism Diagonal new [Re: george tatsis]
      #6232726 - 12/04/13 05:20 AM

I remember someone mentioning ,about two years ago, that the Vixen prism diagonal was pretty good in his scopes.

I think it was Jim, but not 100% sure about it.

Anyway, has anybody ever had any experience with this particular diagonal?

http://www.vixenoptics.com/acc/indiv/3675.html

George


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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6233578 - 12/04/13 03:25 PM

Quote:

I had the opportunity last evening for a first look evaluation of a Prism Diagonal vs. a Dialectric Mirror diagonal. The Prism was the Tak 1.25" (on loan) and the Dialectric was the AstroTech 1.25". Scope used was the TSA-102 and just two targets, Jupiter and M42. Eyepieces were the 6mm ZAO-II and the 5mm XO for planetary. For M42 Sterling Plossls were used, somtimes Barlowed.

Bottom line was that the Prism won the show on both targets, producing a definite notch better results on Jupiter and being subtly better on M42.



As I would expect given the comparison between a top-notch prism and a mediocre mirror diagonal. The results might have been different with a top-notch mirror diagonal, like an AP or TeleVue.
Quote:


The Tak Prism provided a clearer view of a white spot transit, showing greater perceived contrast as well as crisper definition. Structure within the main belts similarly had greater perceived contrast and crisper definition, with more detail being visible as a result. The polar region differences were even more pronounced. In the Prism there was a lot of detailed structure visible instead of just being a simple gradient of shades. The Dielectric showed none of this. Scatter around Jupiter appeared at about the same extent in both diagonals, however it was not as bright in the Prism. Finally, and oddly IMO, the focus snap was much more pronounced using the Prism. Overall I felt like these two eyepieces were performing at what I would characterize as a level better in planetary performance, similar to what I might see going from a lesser performing eyepiece to the ZAOs.



The results would also have been different on a shorter f/ratio scope.
Quote:


Moving to M42 I was looking for three things: faintest star transmission, nebula extent, and mottled structure performance. With transmission, both diagonals were performing the same relative to stars. In both Trap-E was just as well defined (F was not showing this evening) and averted vision stars were showing just as well. Extent of the nebula was slightly better in the Prism, with the outer wings of M42 showing well with averted vision in the Prism and barely at all with the Dielectric. The brightness of M43 was also more obvious in the Prism vs the Dielectric. Finally, the mottled structure presented itself close to the same with each diagonal, but my impression was that it was just a little more detailed with the Prism. This only became somewhat apparent when the magnification was increased -- not apparent using the 17mm (48x) but then seemingly better with the 2.5x Barlowed 25mm (82x). So not a firm call here.

As a side note, I do have an old Celestron 1.25" Prism from the 1980's, but wanted to see how a quality modern Prism performs against a modern Dielectric. FWIW, a surprise for me was that the light path of the Tak Prism was fairly shorter than the old Celestron Prism. Something I was not expecting.



The light path may be an indication of a slightly smaller prism used.
In general, a prism starts introducing lateral chromatic aberration in the shorter focal ratios, but at the ratio you tested it's negligible.
I did a test of a $50 multi-coated prism (plastic housing, shudder) with a high-end dielectric (note spelling) diagonal and found the prism stood up to the dielectric in every characteristic except construction quality. With Baader or Takahashi, though, the construction quality is up there, so a prism makes a reasonable alternative to a mirror. With 3 optical surfaces versus one, the prism has to be made to a high standard.
I'd really like to see a multi-coated prism with a protected silver coated diagonal surface. Light transmission would be superb and optical quality could be really high.
The spectrum of transmission for prisms is a little different than a mirror, but because most of the differences occur at longer wavelengths, where our eyes are less sensitive, they don't matter much in the field.
For a manufacturer, though, a high end prism is a lot more expensive to make: more glass, 3 surfaces to figure, and equal coating costs (or higher).


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Re: Eyepiece Performance using a Prism Diagonal new [Re: Starman1]
      #6233619 - 12/04/13 03:53 PM

Quote:

Quote:

I had the opportunity last evening for a first look evaluation of a Prism Diagonal vs. a Dialectric Mirror diagonal. The Prism was the Tak 1.25" (on loan) and the Dialectric was the AstroTech 1.25". Scope used was the TSA-102 and just two targets, Jupiter and M42. Eyepieces were the 6mm ZAO-II and the 5mm XO for planetary. For M42 Sterling Plossls were used, somtimes Barlowed.

Bottom line was that the Prism won the show on both targets, producing a definite notch better results on Jupiter and being subtly better on M42.



As I would expect given the comparison between a top-notch prism and a mediocre mirror diagonal. The results might have been different with a top-notch mirror diagonal, like an AP or TeleVue.




Very valid point here. However, I do have my reservations. At planetary magnifications I work at sub 1mm exit pupils routinely. At these magnifications, as RC says, only a very small portion of the mirror is used so the actual P-V is likely to be exceedingly small. And given that the focal plane is so close to the mirror, any aberration excepting astigmatism will really be inconsequential visually. At least, this is what he says. However, if someone wants to loan me a TV or AP diagonal I would be happy to add them to the tests

btw, what about quartz diagonals? Many contend best contrast from these period! Thoughts? As far as brightness I read that the TV edges the Vernonscope Quartz, but does it also edge the AP for brightness?

ps - here's an interesting exchange - link.


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Re: Eyepiece Performance using a Prism Diagonal new [Re: jrbarnett]
      #6233655 - 12/04/13 04:22 PM

Quote:

However, there is no question that prism diagonals introduce slight overcorrection and mirror diagonals do not, so assuming a refractor is not under-corrected, a prism introduces spherical aberration whereas a mirror does not.




If the intent is to use a prism diagonal to eke out that last bit of constrast in planetary observation, chances are you're already using a barlow to boost magnification. If so, a simple way to reduce the impact of the prism on color and spherical aberration when using it in faster scopes is to put the barlow ahead of the prism diagonal. It "narrows" the light cone and reduces the effect of the prism on the correction of the scope. Such a barlow also magnifies more than its nominal value so that's an added benefit. With such a setup, I can see no difference between Baaders's T2 prism and Maxbright diagonals in terms of spherical or chromatic correction.

Tanveer.


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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6234167 - 12/04/13 08:41 PM

Quote:


btw, what about quartz diagonals? Many contend best contrast from these period! Thoughts? As far as brightness I read that the TV edges the Vernonscope Quartz, but does it also edge the AP for brightness?

ps - here's an interesting exchange - link.



In theory, quartz can be polished smoother than glass.
In practice? Who knows?
If the Vernonscope Quartz is a standard coating, for sure the TV dielectric version will be brighter. The AP MaxBright is also a dielectric unit, though, so I wouldn't expect any difference in brightness at all to anything short of a spectrophotometer.

P.S. I side with Roland Christen in that discussion.

Edited by Starman1 (12/04/13 08:44 PM)


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Re: Eyepiece Performance using a Prism Diagonal new [Re: Starman1]
      #6234954 - 12/05/13 09:28 AM

Quote:

P.S. I side with Roland Christen in that discussion.




I only half side with him. Reason being is that what he states as fact is not so. Clever marketing (he has a product in the mix, GH did not). Silver coatings can easily achieve more than 99% reflectivity. Edmund graphs show them for their non-specialized production as being >98%. As example, the 8m Gemini telescope in the 1990s using magnetron sputtering to deposit the silver achieved 99.4% reflectivity. That was a decade before the conversation linked. Also, a very critical point that was made by someone but the everyone remained silent on was scatter. It's fine for a diagonal to have highest reflectivity and best flatness, but scatter is more important for planetary observing. Today there are lab techniques to measure and quantify this. AP diagonal is claiming 1/20-1/40 wave in the observing region and >99% transmission with no warping and perfect collimation. But if its scatter is twice the level of a 1/10 wave and 98% reflective diagonal as example, this diagonal will win on planetary details. They had a nice although brief debate...lot of posturing...no resolution though

PS - the Vernonscope diagonal that GH refers to is Quartz, Silver, 1/20th wave with individual report proving, and 99% reflective. Also way expensive.

Edited by BillP (12/05/13 09:32 AM)


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Re: Eyepiece Performance using a Prism Diagonal new [Re: John Harold]
      #6235233 - 12/05/13 11:48 AM

If anyone has experience using a prism or Tak prism on a Sky Watcher 100 ED, I sure would like learn what you thought of it compared to a mirror.

Hey, John Harold, do you know if the Orion 100 ED is the same lens as the Sky Watcher 100 ED? Have you tried the prism in your Orion 100? Wad ya think?

Thanks,

Dan

Edited by dcoyle (12/05/13 12:02 PM)


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John Harold
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Re: Eyepiece Performance using a Prism Diagonal new [Re: dcoyle]
      #6236282 - 12/05/13 09:47 PM

Other posts agree the Sky Watcher and the Orion are the same lens (Synta?). I did use the same old Japanese Celestron prism when I used the 100ED. Now the 100 collects dust in the closet since I got the 120. I recall the 100 was really sharp, I spent many evenings investigating Mars. I'm going to set the 100 up for short run star parties. It's much easier to deal with than the 127, and it's feather light.

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Re: Eyepiece Performance using a Prism Diagonal new [Re: Starman1]
      #6236619 - 12/06/13 04:13 AM

Quote:

I'd really like to see a multi-coated prism with a protected silver coated diagonal surface. Light transmission would be superb and...




Don,
I don't understand this sentence. A 90 deg (zenit-) prism made of BaK4 works entirely with total reflection (=> 100% reflection) and certainly does not need any additional reflective coating.
Do I misunderstand anything in any respect?

Chris


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Starman1
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6236972 - 12/06/13 10:05 AM

Quote:

Quote:

I'd really like to see a multi-coated prism with a protected silver coated diagonal surface. Light transmission would be superb and...




Don,
I don't understand this sentence. A 90 deg (zenit-) prism made of BaK4 works entirely with total reflection (=> 100% reflection) and certainly does not need any additional reflective coating.
Do I misunderstand anything in any respect?

Chris



It has been measured that aluminizing or silvering the 45 degree surface improves light transmission through the prism. Apparently, the reflection is not 100% from that surface--both because it is not perfectly flat and because not all light has exactly a 45 degree angle of incidence.


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Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6236978 - 12/06/13 10:09 AM

Hello ,


>>> I don't understand this sentence. A 90 deg (zenit-) prism made of BaK4 works entirely with total reflection (=> 100% reflection) and certainly does not need any additional reflective coating. <<<<<

He may mean using one surface like a Mangin mirror .

Leonard


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Fomalhaut
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Starman1]
      #6237090 - 12/06/13 10:59 AM

Quote:

Quote:

Quote:

I'd really like to see a multi-coated prism with a protected silver coated diagonal surface. Light transmission would be superb and...




Don,
I don't understand this sentence. A 90 deg (zenit-) prism made of BaK4 works entirely with total reflection (=> 100% reflection) and certainly does not need any additional reflective coating.
Do I misunderstand anything in any respect?

Chris



It has been measured that aluminizing or silvering the 45 degree surface improves light transmission through the prism. Apparently, the reflection is not 100% from that surface--both because it is not perfectly flat and because not all light has exactly a 45 degree angle of incidence.




"...and because not all light has exactly a 45 degree angle of incidence." => According to physics, doesn't have to - not even with BK7-prisms, so much the less with BaK4.
Total reflection is total. The range of angles under which it occurs depends on the refractive index of the glass used.

So please give your sources.

Chris


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Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6237582 - 12/06/13 03:08 PM

Edmund will coat the hypotenuse to increase reflectivity:
http://www.edmundoptics.com/optics/prisms/right-angle-image-reflection-prisms/

From MellesGriot:
"The index of BK7 is sufficiently high to guarantee the TIR (total internal reflection) of a collimated beam at 45 degrees internal incidence over the visible and near infrared region. The possibility of significant TIR failure with
convergent or divergent beams should be kept in mind if polarization
is important. TIR can also fail if the hypotenuse face is not
kept extremely clean. Even an almost invisible fingerprint can lead
to TIR failure. An aluminum or silver coated hypotenuse is recommended
for applications where the right angle-prism is frequently
handled, or where convergent or divergent beams are used.

There is a slight loss of reflectance at all internal angles with the coating,
and no critical angle exists."
I also found a hundred other links in a few minutes, but the note from MellesGriot should suffice.
TIR is only 100% at the surface for collimated light at exactly one angle.


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Re: Eyepiece Performance using a Prism Diagonal new [Re: Starman1]
      #6239207 - 12/07/13 02:21 PM

Quote:

Edmund will coat the hypotenuse to increase reflectivity:
http://www.edmundoptics.com/optics/prisms/right-angle-image-reflection-prisms/




=> But not for astronomical 90deg-(zenit-)prisms. Not even Zeiss ever considered doing so to be helpful.

Quote:

From MellesGriot:
a) The possibility of significant TIR failure with convergent or divergent beams should be kept in mind if polarization is important.
b) TIR can also fail if the hypotenuse face is not kept extremely clean. Even an almost invisible fingerprint can lead to TIR failure.
c) An aluminum or silver coated hypotenuse is recommended for applications where the right angle-prism is frequently handled, or where convergent or divergent beams are used.

d) There is a slight loss of reflectance at all internal angles with (!) the coating, ..."




a) => Is it to you?

b) With reasonable care during the manufacturing process and proper housing a zenit-prism will be (and stay) without any fingerprints on the hypotenuse.

c) The convergence of light before entering the eyepiece is only ~1 deg for a ~2 deg TFOV. Lightthroughput for such a case is still far above 99%.

d) Here it becomes quite clear why reflection coating the hypotenuse of astronomical zenit-prisms is not applied for standard use by any producer. Or is it? (Well, might be for infrared)

Chris


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Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6239860 - 12/07/13 09:13 PM Attachment (21 downloads)

I did a little shootout last night (or attempted at least). Conditions were suboptimal, with less than ideal seeing, and thin changing cloud cover and 20 degrees temperature with me standing in snow.
I used a Starfire 152 with Jupiter as the target. I used a Pentax 5 mm XW and 5 mm XO (more time with the XW). Contenders were a Baader 2 inch dielectric, a Baader 2 inch prism (with Zeiss prism) and a Vernonscope gold.

I saw no striking difference between the two Baaders under these conditions (with brief moments of reasonable seeing and cold feet). Sharpness seemed the same. I did not see color fringing with the prism. The Vernonscope seemed just as sharp to me, but with the gold diagonal it is like a planetary filter and seem to complement the appearance of Jupiter and may have made subtle cloud band more conspicuous.
Here is the clear difference that I did not expect. The Vernonscope is clearly the smaller built diagonal, yet the Baader prism had the shortest optical path, even with the longer (non Clicklock) eyepiece holder. I looked on the draw tube measurement on my focuser. The Baader dielectric came to focus at 49 mm , Vernonscope Gold at 59 mm, and Baader prism at 62 mm. This actually makes a difference to me, since I could not reach focus with the Baader dielectric when using my 5 inch telescope with the 0.75x focal reducer and my low power eyepiece. I cold reach focus when using a TV Everbrite with some, but also not all eyepieces.
I am hoping for clearer and warmer weather to spent more time, however I do not have the experience or eye of many keen observer on this forum.


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Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6239861 - 12/07/13 09:14 PM Attachment (23 downloads)

Second image

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Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6241042 - 12/08/13 03:41 PM

I just started some testings with the diagonals I have on hand, thanks to the loaners from folks

Just did some indoor tests to measure their light paths of the diagonals only, without any 2" to 1.25" converters if they needed that, Here's the results of the measures:

MM -- DIAGONAL
51.9 -- Tak 1.25" Prism
53.3 -- Celestron 1980 Prism
62.5 -- Baader T2 Prism*
81.2 -- AT 1.25 Dielectric Mirror
84.3 -- Baader 2" Prism
96.1 -- AT 2" Dielectric Mirror

These raw numbers are a little deceptive as the Baader T2 is configured with a 2" nosepiece and a 1.25" eyepiece holder so it does NOT need a 2"-to-1.25" adapter anyplace in the train when used with a scope with a 2" focuser and a 1.25" eyepiece. So for the actually operating backfocus one needs to add the length of the 2"-to-1.25" adapter to these numbers for total operational backfocus (except for the Baader T2). I have several of these converters, the shortest adds 9.6mm, but others I have are 12.4mm. So the Baader T2 and the Tak fight with each other as to which needs the least backfocus. If I use the 9.6mm length adapter with the Tak prism then it is slightly shorter overall than the Baader, if I use the 12.4mm adapter then the Baader T2 is shortest of the bunch.


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Re: Eyepiece Performance using a Prism Diagonal new [Re: urassner]
      #6241307 - 12/08/13 06:17 PM

urassner: Because the apparent thickness of the prism or the "air equivalent" path length is the actual path length divided by the glass refractive index, your results are just what should be expected. For example, assuming a n = 1.5 prism glass index, if a prism in-glass path length was 100 mm, the air equivalent prism path length would be 100mm/1.5 = 66.7 mm. So the light path in the prism would take up only 66.7mm of the focal distance. (Even though it measures 100 mm by a ruler alongside the prism!) Thus the location of the image plane when using the prism diagonal would be moved back by an extra (100 mm- 66.7mm) = 33.3 mm as compared to a non-refractive mirror diagonal. The bigger the prism (i.e. longer in-glass path), the farther back the image plane will be.

Folks are often confused by this because adding a prism erecting system, as in binoculars, often seems to move the eyepiece closer to the objective. But this is only because of the "folding" reflections of the light path within the prism cluster. When focusing light travels within prism glass, the effect is to move the location of focus farther away from the objective. Hence, the thicker your prism, the farther back your eyepiece will focus. This idea is why a telescope without sufficient "in" focus to accommodate a particular eyepiece when a mirror diagonal is used can often successfully focus with the same eyepiece when a prism diagonal is substituted.

BillP : How did you get your diagonal light path measurements using the diagonal only? A mirror diagonal's path length can be directly measured on the diagonal, but I do not understand how you measured the prism diagonal light path length without either calculation with refractive index or using a scope and eyepiece. Maybe I'm missing something in understanding your method. Thanks, BillB


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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillB9430]
      #6241340 - 12/08/13 06:46 PM

Quote:


BillP : How did you get your diagonal light path measurements using the diagonal only? A mirror diagonal's path length can be directly measured on the diagonal, but I do not understand how you measured the prism diagonal light path length without either calculation with refractive index or using a scope and eyepiece. Maybe I'm missing something in understanding your method. Thanks, BillB




I measured it operationally. So the scope was focused on an object with no diagonal using an eyepiece where there is no offset of the field stop from the shoulder. Length of focusers extension was then measured. Diagonal was put in place and the same eyepiece used, refocused, then focuser extension re-measured. Difference was the amount of backfocus being used by the prism and its fixed housing components...minus the amount being used by the 2-to-1.25: adapters since not all the units needed these.


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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6241466 - 12/08/13 08:02 PM

Thanks Bill, that makes sense. I somehow got the incorrect notion that you measured indoors without a telescope and could not figure out how that could be done. The Baader 2" prism diagonal's position seems an anomaly, but perhaps is due to the larger prism requiring that the eyepiece be farther from the principal axis? It certainly is in proper position relative to the 2" mirror diagonal. - BillB

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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillP]
      #6241776 - 12/08/13 10:58 PM

My conclusions match yours Bill. Good work.

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Re: Eyepiece Performance using a Prism Diagonal new [Re: Daniel Mounsey]
      #6241780 - 12/08/13 11:01 PM Attachment (18 downloads)

BTW, Vernonscope Silver and Gold we discussed today Bill.

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Re: Eyepiece Performance using a Prism Diagonal new [Re: BillB9430]
      #6242575 - 12/09/13 12:38 PM

Hi BillB9430, thanks for the explanation. It makes perfect sense. I just had never thought about that aspect. I am surprised though, that this is not mentioned as a marketing feature of the prism diagonal. In fact, Baader lists the optical path length for some diagonals on the is website, but not for this one. Compared to my Baader Dielectric, the optical path was 13 -4 mm shorter and that was despite the longer eyepiece holder used on the prism diagonal. I would think that for some telescopes this shorter light path could make all the difference when using binoviewers or focal reducers (it does to me with the focal reducer, but I have not tried the binoviewer).

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Starman1
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Re: Eyepiece Performance using a Prism Diagonal new [Re: Fomalhaut]
      #6242858 - 12/09/13 03:05 PM

Quote:

Quote:

Edmund will coat the hypotenuse to increase reflectivity:
http://www.edmundoptics.com/optics/prisms/right-angle-image-reflection-prisms/




=> But not for astronomical 90deg-(zenit-)prisms. Not even Zeiss ever considered doing so to be helpful.

Quote:

From MellesGriot:
a) The possibility of significant TIR failure with convergent or divergent beams should be kept in mind if polarization is important.
b) TIR can also fail if the hypotenuse face is not kept extremely clean. Even an almost invisible fingerprint can lead to TIR failure.
c) An aluminum or silver coated hypotenuse is recommended for applications where the right angle-prism is frequently handled, or where convergent or divergent beams are used.

d) There is a slight loss of reflectance at all internal angles with (!) the coating, ..."




a) => Is it to you?

b) With reasonable care during the manufacturing process and proper housing a zenit-prism will be (and stay) without any fingerprints on the hypotenuse.

c) The convergence of light before entering the eyepiece is only ~1 deg for a ~2 deg TFOV. Lightthroughput for such a case is still far above 99%.

d) Here it becomes quite clear why reflection coating the hypotenuse of astronomical zenit-prisms is not applied for standard use by any producer. Or is it? (Well, might be for infrared)

Chris



A) We do have convergent rays coming through the diagonal, so light throughput and the spectrum of transmission is important.
B) Polarization is important in planetary viewing. Use a single polarizer on Jupiter and rotate it. Interesting, eh?
C) extreme cleanliness of the 45 degree surface is guaranteed if it is coated.
D) The percentage loss with coating is negligible, but phase coherence is maintained.

At any rate, the internal reflection is not 100% except for a particular wavelength at exactly the correct angle of incidence. Prisms also add loss due to the thickness of glass and the spectrum of transmission. In an f/15 scope, though, a prism can be a reasonable alternative to a mirror system. At f/5-6, not so much.


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UPDATE: Prism Diagonals new [Re: BillP]
      #6247933 - 12/11/13 09:45 PM

UPDATE:

Just did a round of color tests with the diagonals. Used a very difficult target, the bright limb of the Moon placed dead center in the AFOV.

Scope, a fast f/6.25 APO, the Celestron Onyx 80mm. Chose this one first to see what kind of color I could tease out since it was a fast focal ratio and folks have been saying this is where the prisms do not operate well.

Eyepieces chosen were a 6mm ZAO-II and a 4mm TMB Monocentric which produce 83x and 125x respectively. Then pumped things up and used the 6mm ZAO-II with the APM 2.7x ED Barlow for 225x (.35mm exit pupil and 71x per inch aperture).

Results....

#1 -- A-T 1.25" Dielectric (no color)
#2 -- Baader T2 Prism (V. Slight)
#3 -- Celestron 1980 Prism (Slight)
#4 -- Tak Prism (Slight+)
#5 -- Baader 2" Prism (Slight++)

Not the greatest ranking words, but for the moment all I can think of. Basically it was a difficult observation to see the color once in focus with #2 and #3. Dark shadows in craters were just a little off-black, difficult to see a slight blue tinge to the black shadows. #4 it was more evident that the shadows were not completely black and definitely blue-black, and in #5 they were more obvious still having the blue tinge to the shadows. #5 is the only one also where the slight blue/purple halo off the limb was of any size. In all the others any color showing on the limb was infinitesimally thin and small when in focus.

At highest magnification, 225x out of the 80mm is where the crater shadows showed their blue-black appearance. Working at 83x and 125x pretty much everything looked black in the shadows across the Lunar surface. Image was also completely sharp and etched at 225x out of this little 80mm (I really love this scope...it is an excellent specimen and had no issues keeping up with the LOMO 80mm I used to have). I was beginning to do an assessment of how sharply the image appeared through each diagonal at the 225x magnification but only got one round in before cloud deck came through. So I have no ranking yet for who was the sharpest of the bunch but the bottom dweller was very evident....the A-T 1.25" Dielectric was no where in the same league as the Prisms! While the prisms showed a sharply etched view, the Dielectric was soft in comparison. However, I have some premium mirrors on the way to add to the comparison so will redo all these tests once they arrive in this same scope, then move to the TSA-102 to see how things change at f/8. Bottom line though, residual color was really slight and unobtrusive at f/6 IMO.


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leonard
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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6248137 - 12/11/13 11:49 PM



Quite interesting Bill , keep up this great review.

Leonard


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george tatsis
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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6248228 - 12/12/13 01:13 AM

Keep up the good work Bill, we're counting on you

God knows how many eyepieces over the years have been reported to be insufficient because of light scattering diagonals used with them.

I think we, planetary observers, should first pick our diagonals and then our eyepieces

I've placed an order for the Vixen 1.25" prism diagonal to compare it with the current WO dielectric that I use in my 120ED, and I'll report back when some solid conclusions are drawn.

An order has also been placed for the GSO 1.25" non dielectric mirror diagonal to compare the views as well.

For some reason I can see myself in the future ditching the dielectric one for planetary observing because it DOES scatter light.

I should have thought of this issue years ago when my buddy's SW 80 ED consistently outperformed my TV 85 by exhibiting darker background, less scatter, and darker Jovian belts.

At that time it didn't cross my mind that his $35 diagonal would be better for Jupiter than my Everbrite, but it was.

George

Edited by george tatsis (12/12/13 03:51 AM)


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Re: UPDATE: Prism Diagonals new [Re: george tatsis]
      #6248389 - 12/12/13 06:55 AM

Here is a good way to avoid all the controversy about diagonals: Use a Newt!


Mike


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george tatsis
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Re: UPDATE: Prism Diagonals new [Re: Sarkikos]
      #6248400 - 12/12/13 07:10 AM

Quote:

Here is a good way to avoid all the controversy about diagonals: Use a Newt!


Mike






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BillP
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Re: UPDATE: Prism Diagonals new [Re: george tatsis]
      #6248536 - 12/12/13 08:58 AM

Quote:

Quote:

Here is a good way to avoid all the controversy about diagonals: Use a Newt!


Mike











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Starman1
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Re: UPDATE: Prism Diagonals new [Re: george tatsis]
      #6249215 - 12/12/13 02:28 PM

It is possible that there is a little more variation in the optical surface quality in mirror diagonals than there is in prisms. I would think a planetary observer would be concerned with optical quality of the mirror.

In my experience, there is less variation in the higher-end star diagonals (AP, TeleVue, Lumicon, et al.) than there is in the economy ones, though there is some. I had occasion to compare a dozen Lumicons, and I kept one that was obviously better than the others. It was an "enhanced" coated mirror.

If you look at interferogram reports on mirrors used in diagonals, it probably would be a good thing to use a 2" star diagonal, even if using 1.25" eyepieces, because the center 1/3 of every star diagonal's mirror has the least deviation from a perfect surface.

It might even be worthwhile to invest in a really high-accuracy secondary mirror from someone like Antares optics or Protostar to use that mirror as a star diagonal's mirror to get an even better surface.

As for coatings, it does depend a lot on the quality of the coatings, but if you're concerned, a simple aluminizing with SiO overcoat might be best. You certainly won't miss the light, and the surface is likely to be less affected by the coating.

However, Roland Christen seems to think a dielectric-coated mirror is just fine for planetary viewing in an AP scope. He does say this, though:
"For critical planetary viewing it is important to have the eyepiece and diagonal surfaces as clean as possible. You will find that even after one observing session the diagonal surface will accumulate crud. The inherently high contrast and cleanability of the Maxbright insures that it is never a limiting factor when viewing subtle planetary detail."[this would be true of all dielectric-coated diagonals]

Like many labs and professional observatories, he has found that cleanliness means more than coating type when it comes to planetary details viewing. How many of you have cleaned your star diagonals regularly (like every 6 months or more)?


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Roy McCoy
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Re: UPDATE: Prism Diagonals new [Re: george tatsis]
      #6249216 - 12/12/13 02:28 PM

Quote:

I should have thought of this issue years ago when my buddy's SW 80 ED consistently outperformed my TV 85 by exhibiting darker background, less scatter, and darker Jovian belts.




I've seen the same thing with the Orion 80ed and a T2 prism.


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Re: UPDATE: Prism Diagonals new [Re: Roy McCoy]
      #6249330 - 12/12/13 03:18 PM

Yup...the more I am using that T2 Prism, the more it seems to be distinguishing itself from the field.

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Re: UPDATE: Prism Diagonals new [Re: Starman1]
      #6250035 - 12/12/13 10:31 PM

Quote:

How many of you have cleaned your star diagonals regularly (like every 6 months or more)?





Every time I do serious planetary observing, I clean my diagonal.

I give everyone that asks about getting a dielectric diagonal to get one, and I explain that the main reason you should use a dielectric is simply because you can clean it for every serious planetary observing session and not damage it.

And this is one of the things I dislike about prisms. While the coatings on prisms are pretty hard (not as hard as dielectric), they are much harder to clean than a mirror.


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pga7602
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Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6250222 - 12/13/13 01:55 AM

If I wanted to try a 1.25 diagonal in my TV85, is it just a matter of sticking a 1.25 eyepiece adapter ahead of the diagonal? Or is there a diagonal 2-1.25 adapter made for this purpose.

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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6250313 - 12/13/13 05:46 AM

Quote:

Yup...the more I am using that T2 Prism, the more it seems to be distinguishing itself from the field.




Bill,
Since thera are three versions of the T2 prism, I would like to ask you which it is that you have:
http://www.baader-planetarium.de/sektion/s28/s28.htm#t2_90_grad_ohne
Chris


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Re: UPDATE: Prism Diagonals new [Re: Fomalhaut]
      #6250417 - 12/13/13 08:13 AM

It's on loan to me so don't know off the top of my head. Will check this evening to see if something distinguishing marked on the prism that indicates which it is.

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Ava
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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6250449 - 12/13/13 08:44 AM

As has been discussed numerous times before on this forum, the amount of color that shows with any given prism / refractor combination will depend of the interplay of the chromatic aberrations from both sources. Some prisms will cancel out some of the coloring, while some will add to it. Many times it has been suggested that many doublets might benefit from a non-premium prism diagonal, e.g. the Baader T2 non-Zeiss prism, because the prism may cancel out some of the chromatic aberrations of the scope, while a good triplet will have very little color and so needs a premium prisms that introduces none either, e.g. the Baader T2 Zeiss prism. Because we're talking about color, it likely also depends on the color ranges observed. This will obviously need testing for every specific diagonal/objective/target combination.

In conclusion it's likely that you probably need to try for yourself or hear from someone that has the same scope and target preferences as yourself before buying a specific brand of diagonal, at least as far as color is concerned.

Now, regarding scatter, that's another matter and a good prism will probably win over any decent mirror diagonal. Now, when you get to the excellent (and clean!) mirror diagonals, that's where it gets interesting I guess. I'm eagerly awaiting more results!


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Ava
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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6250454 - 12/13/13 08:48 AM

Bill, if it has no markings you could check the optical path length, the Zeiss version is supposed to have around 42mm length while the non-Zeiss should have around 38mm, if I remember correctly.

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Re: Eyepiece Performance using a Prism Diagonal new [Re: junomike]
      #6250480 - 12/13/13 09:04 AM

I have always preferred A mirror diagonal for Deep Space viewing and Prism for Planetary. I have a 10th wave enhansed Lumicon I have owned for decades, and my reysty mis level Prisms. I believe the Mirror to be a bit brighter but prism knocks glare and scattered light down better producing higher detail.

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Fomalhaut
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Re: UPDATE: Prism Diagonals new [Re: Ava]
      #6250650 - 12/13/13 10:35 AM

Quote:

As has been discussed numerous times before on this forum, the amount of color that shows with any given prism / refractor combination will depend of the interplay of the chromatic aberrations from both sources. Some prisms will cancel out some of the coloring, while some will add to it. Many times it has been suggested that many doublets might benefit from a non-premium prism diagonal, e.g. the Baader T2 non-Zeiss prism, because the prism may cancel out some of the chromatic aberrations of the scope, while a good triplet will have very little color and so needs a premium prisms that introduces none either, e.g. the Baader T2 Zeiss prism. Because we're talking about color, it likely also depends on the color ranges observed. This will obviously need testing for every specific diagonal/objective/target combination.





According to Rutten vanVenroij 1st ed.p.312-313, "a reflecting prism behaves as a plan-parallel plate, ...causing overcorrected spherical aberration."
Well reputed (even top-notch such as Tak) apos are slightly undercorrected, presumably i.o. to work better with zenit-prisms. => The prism's overcorrection then tends to neutralize the objectives' undercorrection.
On astro-foren.de, Taks and Zeiss APQs and others have been tested with and without prisms and many of them work better with and others without prisms. Zeiss even produced some of their famous APQs to be used with prisms and others (of same type, aperture and f-ratio) to be used with mirrors.
Mr. Rohr found their measured system-correction was indeed in accordance with the goal of their design.
Chris


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Starman1
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Re: UPDATE: Prism Diagonals new [Re: Fomalhaut]
      #6250765 - 12/13/13 11:38 AM

Quote:


According to Rutten vanVenroij 1st ed.p.312-313, "a reflecting prism behaves as a plan-parallel plate, ...causing overcorrected spherical aberration."
Well reputed (even top-notch such as Tak) apos are slightly undercorrected, presumably i.o. to work better with zenit-prisms. => The prism's overcorrection then tends to neutralize the objectives' undercorrection.
On astro-foren.de, Taks and Zeiss APQs and others have been tested with and without prisms and many of them work better with and others without prisms. Zeiss even produced some of their famous APQs to be used with prisms and others (of same type, aperture and f-ratio) to be used with mirrors.
Mr. Rohr found their measured system-correction was indeed in accordance with the goal of their design.
Chris




I'm sorry. Sometimes a post just goes a little too far off into La-La Land for me to let it go.

1) No manufacturer would purposely build undercorrection into its scopes. They couldn't know if the instrument was going to be used with a prism, a mirror, a camera, or straight through (common in Japan). IF undercorrection is present, it is because the lenses weren't made or spaced correctly.

2) Ditto on the Zeiss refractors.

3) Rutten and vanVenrooij's discussion of prisms referred to oblique angles through a prism, i.e. short f/ratios. It's fairly well known that prisms aren't suitable for short f/ratios due to, well, "the prismatic effect". They mention that a weak positive lens in front of the prism can offset that. This is also well known.


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pga7602
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Re: UPDATE: Prism Diagonals new [Re: Fomalhaut]
      #6250877 - 12/13/13 12:46 PM

If I wanted to try a 1.25 prism diagonal in my TV85, is it just a matter of sticking a 1.25 eyepiece adapter ahead of the diagonal? Or is there a diagonal 2-1.25 adapter made for this purpose.

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Fomalhaut
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Re: UPDATE: Prism Diagonals new [Re: Starman1]
      #6250913 - 12/13/13 01:18 PM

Quote:

Quote:


According to Rutten vanVenroij 1st ed.p.312-313, "a reflecting prism behaves as a plan-parallel plate, ...causing overcorrected spherical aberration."
Well reputed (even top-notch such as Tak) apos are slightly undercorrected, presumably i.o. to work better with zenit-prisms. => The prism's overcorrection then tends to neutralize the objectives' undercorrection.
On astro-foren.de, Taks and Zeiss APQs and others have been tested with and without prisms and many of them work better with and others without prisms. Zeiss even produced some of their famous APQs to be used with prisms and others (of same type, aperture and f-ratio) to be used with mirrors.
Mr. Rohr found their measured system-correction was indeed in accordance with the goal of their design.
Chris




I'm sorry. Sometimes a post just goes a little too far off into La-La Land for me to let it go.

1) No manufacturer would purposely build undercorrection into its scopes. They couldn't know if the instrument was going to be used with a prism, a mirror, a camera, or straight through (common in Japan). IF undercorrection is present, it is because the lenses weren't made or spaced correctly.

2) Ditto on the Zeiss refractors.

3) Rutten and vanVenrooij's discussion of prisms referred to oblique angles through a prism, i.e. short f/ratios. It's fairly well known that prisms aren't suitable for short f/ratios due to, well, "the prismatic effect". They mention that a weak positive lens in front of the prism can offset that. This is also well known.





1) Before introducing their FSQs Takahashi designed their Fluorite FCs and FSs mainly for the (visual) use with their own dedicated zenit-prisms. Together with the FSQs they introduced an expensive and extremely accurate short-building 2-inch mirror-diagonal (for low-straylight applications deliberately and explicitely renouncing dielectric coatings).

2) I suggest some people perhaps write a little less and read a little more, e.g.
http://www.astro-foren.de/showthread.php?6084-Verzeichnis-optischer-Berichte
on several APQs with or without glass path

3) Yes, very well known over here in LaLa-land .
I bought and started reading that book over and over in 1990.
Nevertheless there were chromatic differences visible in my former f8.8 Vixen-Fl-102 with/without zenit-prism...

Chris


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Re: UPDATE: Prism Diagonals new [Re: pga7602]
      #6250951 - 12/13/13 01:38 PM

Quote:

If I wanted to try a 1.25 prism diagonal in my TV85, is it just a matter of sticking a 1.25 eyepiece adapter ahead of the diagonal?




Yup.


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pga7602
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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6250998 - 12/13/13 02:03 PM

Thanks Bill.

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Ant78
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Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6251197 - 12/13/13 03:56 PM

Quote:

Quote:

How many of you have cleaned your star diagonals regularly (like every 6 months or more)?





Every time I do serious planetary observing, I clean my diagonal.

I give everyone that asks about getting a dielectric diagonal to get one, and I explain that the main reason you should use a dielectric is simply because you can clean it for every serious planetary observing session and not damage it.

And this is one of the things I dislike about prisms. While the coatings on prisms are pretty hard (not as hard as dielectric), they are much harder to clean than a mirror.



I recently used the method you suggest on my dielectric diagonal and it worked a treat, cheers edgie


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Re: UPDATE: Prism Diagonals new [Re: Fomalhaut]
      #6251693 - 12/13/13 08:42 PM

Quote:

Quote:

Yup...the more I am using that T2 Prism, the more it seems to be distinguishing itself from the field.




Bill,
Since thera are three versions of the T2 prism, I would like to ask you which it is that you have:
http://www.baader-planetarium.de/sektion/s28/s28.htm#t2_90_grad_ohne
Chris




According to the owner, both Baader prisms are the Zeiss prisms. The T2 is fitted with the 1.25" eyepiece clamp and a 2" nose piece. I will take some pics tomorrow when get some daylight and post.


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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6252371 - 12/14/13 10:06 AM

There are people that think there T2 diagonal has a Zeiss prism, but there are indeed two very different T2 Prism diagonals sold and only one of them has a Zeiss prism.

The $129 unit does not have a Zeiss prism. People often get the impression that it does becuase some of the sales info posted on vendor pages is cut and pasted from Alpine Astro or Baader and taken out of context of the full Baader writeup.

When one reads the full writeup on the Alpine Astro website, all of the writeup is for the Zeiss prism.

But when you go to the order page, you see that the standard prism is only 32mm of clear apture, while the Zeiss prism unit has 34mm (I have the standard Priam and it is only 32mm apeture). And on the price page, it specifices that the T2 01B has a Zeiss prism, but makes no such claim for the T2-01C.

Also, the light path is considerably shorter in the standard prism vs the Zeiss prism unit.

I have addressed this many times on the binoviewer forum where people that had the T2-01C ($129) diagonal thought that it had a Zeiss prism, but nowhwere will you find this documented. Almost all of the sales data that says the $129 unit has a Zeiss prism I beleive is in error and has used the writeup for the much more expenisve Zeiss unit.

I do not think this is any attempt to deceive anyone. I think it is an honest mistake.

But I have looked all over for it, and in no instance on any Baader literature has it said that the T2-01C has a Zeiss prism, but it always says that for the T2-01B.

So, a lot of people seem to think that their $129 diagonal has a Zeiss prism, but it does not.

Not sure what your friend has. Maybe he has the T2-01B. Maybe not.

But I know a lot of people thought they bought the T2-01C beliveing they had a Zeiss prism.

And I have to say this as well.

I have the T2-01C and I also have the Max 1 Maxbright Dielectric as shipped with the Mark V binoviewers.

I have compared the performance of both diagonals in my EdgeHD 8" and found it difficult to tell them apart. I would say that even the $129 diagonal is a really excellant quality part.

I also had the Tak 1.25" prism, and I never found it to be all that. Compared it to a 2" Televue dielectric and found it to be about the same in performance.


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Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6252412 - 12/14/13 10:31 AM

Quote:

When one reads the full writeup on the Alpine Astro website, all of the writeup is for the Zeiss prism.

But when you go to the order page, you see that the standard prism is only 32mm of clear apture, while the Zeiss prism unit has 34mm (I have the standard Priam and it is only 32mm apeture). And on the price page, it specifices that the T2 01B has a Zeiss prism, but makes no such claim for the T2-01C.




I've also noticed this. If a company - or private seller on CN or AM, for that matter - does not say specifically that a product has such-and-such a quality or includes this-or-that, it's much safer to assume that it doesn't. Even if the product is said to have this-or-that specialness, be skeptical if the price seems too good to be true.

Mike


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Roy McCoy
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Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6252470 - 12/14/13 11:19 AM

Quote:

So, a lot of people seem to think that their $129 diagonal has a Zeiss prism, but it does not.




I would think less than half the price would tip people off to the fact that it does not have the Zeiss prism – it did for me. As Mike says above "if the price seems too good to be true".

I’ve ordered from Alpine and you have to send an email so clicking the wrong box is not a possibility.

Maybe Bob will read this thread and add the designation (Baader Prism) to the T2-01C.

Edited by Roy McCoy (12/14/13 11:23 AM)


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Don Taylor
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Re: UPDATE: Prism Diagonals new [Re: Roy McCoy]
      #6252537 - 12/14/13 12:04 PM

I'm looking to get a Baader T2-01C as well and understood that it does not have a Zeiss prism - but the short optical path solves some unique issues for my application.

BTW - it is also available from Agena (along with the other Baader diagonals)- although neither Apline nor Agena have any in stock at this time.


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BillP
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Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6252672 - 12/14/13 01:20 PM

Interesting Eddgie. Important for people to take note of when purchasing.

I removed the 1.25" eyepiece adapter so I could get nearer the prism, then illuminated a pattern laser throught the other end and measured the illuminated FOV and got 34.2mm. Plus the purchase prise was way more than $129 So given both these facts, Zeiss I'd say.


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Eddgie
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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6253393 - 12/14/13 08:39 PM

Measure on the front side. The rear side of the $129 prism is 34mm, but the front side is 32mm.

Here is the Baader page showing both. They look a lot a like, but the nose opening of the $129 (non Zeiss) is only 32mm...

Baader page showing both diagonals

So, if the front of the unit has a 32mm opening, this is the non Zeiss. The Zeiss is a full 34mm at the front and rear.

The non-Zeiss is 32mm at the front and 34mm at the rear, but the front opening determines the true aperture.

So, measure at the front to know for sure.

And notice again, on the Baader page, they are very clear that the T2 01B has the Zeiss prism, but no mention is made of having a Zeiss prism in the standard diagonal.

Not saying you do or don't have one.

My main point here is that everyone thinks that the standard T2 diagonal has a Zeiss prism and nowhere is that ever mentioned on the Baader page.

But it does show up on a lot of web sites and again, I think they have made an honest error and simply used the T2-01B sales info by mistake.

I want to be sure to re-enforce this again. the standard Prism is a superb prism. As best as I can see, identical in performance at f/10 to the Televue and Baader dielectric diagonals.


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Tamiji Homma
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Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6253438 - 12/14/13 09:10 PM

Here is several Baader diagonal side-by-side photos:

From left to right:

Baader T2-01C prism, T2 mirror, Amici prism, T2-01B (Zeiss), Prism-2 (2"):




Here is T2-01B (Zeiss) and Takahashi prism side-by-side:


Tammy


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leonard
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Re: UPDATE: Prism Diagonals new [Re: Tamiji Homma]
      #6253540 - 12/14/13 10:11 PM

Hello Tammy ,


It looks to me that the diagonal has screws to let one remove the prism from the housing for cleaning , is this true ?
It was mention that prisms are a little hard to clean , have you done it ?

Thanks for any info. Leonard


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Roy McCoy
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Re: UPDATE: Prism Diagonals new [Re: leonard]
      #6253558 - 12/14/13 10:26 PM

Quote:

It looks to me that the diagonal has screws to let one remove the prism from the housing for cleaning , is this true ?





Yes it is, and you can, and I have.


From Ed's link above and from Tammy's picture, it looks as though the T2/Zeiss has a silkscreened logo while the T2/Baader has an embossed logo.


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BillP
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Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6253622 - 12/14/13 11:21 PM

Man...you guys are such doubting thomases Beating this issue to death.

OK. The laser pattern method very accurately projects the 34mm stop of the prism as I did it the last time. However, just to reinforce this, I directly measured that aperture stop opening at both front and rear, top and bottom, left and right, up and down (depends on how you orient the thing ) and.....

....drum roll.....

....just as the owner said and just as the laser projection produced....

all apertures are 34mm

ZEISS!!!!!!!!!!!!!!!!!!!!!!!!!!



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warpsl
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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6253813 - 12/15/13 03:34 AM

hello.my 2 cents worth! I have both the baader zeiss 34mm,and the baader 32mm aperture prisms.i actually wanted only zeiss,but I was told they were not available,so initially I bought the jap made baader.then late the zeiss.i use them with my skywatcher 6 inch bino,s.there is no colour on d s o,but there is some on the moon and planets.but this is dependant on the eyepieces I use.the denk 14mm giving the cleanest views.i struggle to see any difference in performance between these 2 prisms.both have excellent anti-reflection coatings,and give excellent contrast.regards.

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Eddgie
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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6254069 - 12/15/13 09:39 AM

Oh forgive me.

I was not doubting you so much as trying to make sure that people knew that the $129 diagonal does not have a Zeiss made prism.

Becuase so many web pages say this, I think everyone believes that the $129 diagonal has Zeiss too.

Did not know what you had. Did not question it so much as to tell you how to know for sure.

But my main point is still this. The $129 digaonal, in spite of the many web pages that say differently, does not have a Zeiss made prism. At least Baader itself does not say so.

I just don't want someone to read your review and go out to buy the $129 diagonal and think they are getting Zeiss prism.

But I do want them to feel confident that the $129 prism is also of excellent optical quality, and if they really want a prism diagonal, they can rest assured that they are getting an excellent one.

I was not all that impressed with the Tak prism. In particular, I hated the compression ring eyepecie holder.

But optically, having owned both, and having compared them all to dielectric diagonals at f/10, I found no difference it quality.

Based on my experience with the f/7 refractor though, I would lean towards a mirror. I know that the prisms in the binoviewer caused pretty severe chromatic aberration at f/7.

The Max mirror though gives amazingly sharp images, and I think perhap at f/7, better than the prism.

Any, my primarly goal was to alert people that the T2 standard does not have the same prism as the T2 Zeiss, and just helping to know how to tell the difference.

Enjoy your testing.


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roadi
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Re: UPDATE: Prism Diagonals new [Re: Tamiji Homma]
      #6254288 - 12/15/13 11:23 AM

Quote:

Here is several Baader diagonal side-by-side photos:

From left to right:

Baader T2-01C prism, T2 mirror, Amici prism, T2-01B (Zeiss), Prism-2 (2"):




Here is T2-01B (Zeiss) and Takahashi prism side-by-side:


Tammy



Nice arsenal Tammy By the way isn't the last one in your picture a zeiss also?
I had a 2" which were a zeiss prism in a baader housing!


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leonard
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Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6254373 - 12/15/13 12:06 PM

Hello ,

>>>> my primarly goal was to alert people that the T2 standard does not have the same prism as the T2 Zeiss, and just helping to know how to tell the difference. <<<<<

Good you pointed this out , no one wants to make a $129.00 mistake.
As always your experience an advice are warmly received .

Leonard


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leonard
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Re: UPDATE: Prism Diagonals new [Re: Roy McCoy]
      #6254376 - 12/15/13 12:08 PM

Hello


Thanks Roy


Leonard


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BillP
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Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6254527 - 12/15/13 01:37 PM Attachment (18 downloads)

Quote:

But optically, having owned both, and having compared them all to dielectric diagonals at f/10, I found no difference it quality.

Based on my experience with the f/7 refractor though, I would lean towards a mirror. I know that the prisms in the binoviewer caused pretty severe chromatic aberration at f/7.




Interesting how everyone's experiences seem to be so very different with prisms. For instance, in my initial tests on the Moon, CA was fairly little to my eye at f/6.25 in the APO. As the weeks roll on will try a variety of targets...so right now have not even stratched the surface of what needs to yet be done.

btw, here's a pic of the current batch. Like I said, some high quality mirror diaginals are on the way so we can have a premium to premium compare as well.

Back Row 2" (L to R): AT 2" Deilectric & Baader 2" Prism

Front Row 1.25" (L to R): Celestron 1980's Prism, AT Dielectric, Tak Prism, Baader T2 Zeiss Prism


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StarDust1
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Reged: 08/21/12

Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6254616 - 12/15/13 02:48 PM

Bill, looking forward to your findings. I just ordered a Tak prism. I'm looking for a 2" mirror or prism. I'm not yet decided. It will be used with a TSA120 and FS60CB.

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SteveC
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Re: UPDATE: Prism Diagonals new [Re: StarDust1]
      #6254848 - 12/15/13 05:12 PM

Bill,

This new project of yours is getting out of hand. Are you going to end up with about 9-10 diagonals? Switching out that many diagonals is a pain in the butt, even in warm weather.


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roadi
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Re: UPDATE: Prism Diagonals new [Re: StarDust1]
      #6254892 - 12/15/13 05:33 PM

Quote:

Bill, looking forward to your findings. I just ordered a Tak prism. I'm looking for a 2" mirror or prism. I'm not yet decided. It will be used with a TSA120 and FS60CB.




Me too about the tests! I have the Tak prism too, I like it very much. To my eyes It handily beated two high quallity dielectric mirror diagonals on planetary contrast.
Rather enhanced mirror diagonal than dielectric


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JustaBoy
Carpal Tunnel


Reged: 06/19/12

Re: UPDATE: Prism Diagonals new [Re: roadi]
      #6254987 - 12/15/13 06:26 PM

I seem to remember reading in one of the old telescope making books, that for the most critical observations a Diagonal is never used...

But then men were a lot tougher back then, or at least in My Case, a Lot Younger! .


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BillP
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Re: UPDATE: Prism Diagonals new [Re: SteveC]
      #6255035 - 12/15/13 07:00 PM

Quote:

Bill,

This new project of yours is getting out of hand. Are you going to end up with about 9-10 diagonals? Switching out that many diagonals is a pain in the butt, even in warm weather.




You are quite right Not an easy or fast process switching diagonals. Any observations relative to sharpness will require seeing that does not fluctuate too much...which means...time waiting that out.

Anyway, was able to conduct a on-axis CA test tonight using the f/8 TSA-102. Conducted as in the previous test using the bright limb of the Moon, basically all the diagonals performed nicely with no hints of color, even racking focus in and out a little. So while the fast f/6.25 showed varying degreees of none to slightly more than minimal color, all showed none at f/8.


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SteveC
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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6255404 - 12/15/13 10:37 PM

Quote:

Quote:

Bill,

This new project of yours is getting out of hand. Are you going to end up with about 9-10 diagonals? Switching out that many diagonals is a pain in the butt, even in warm weather.




You are quite right Not an easy or fast process switching diagonals. Any observations relative to sharpness will require seeing that does not fluctuate too much...which means...time waiting that out.

Anyway, was able to conduct a on-axis CA test tonight using the f/8 TSA-102. Conducted as in the previous test using the bright limb of the Moon, basically all the diagonals performed nicely with no hints of color, even racking focus in and out a little. So while the fast f/6.25 showed varying degreees of none to slightly more than minimal color, all showed none at f/8.




That's great!!
Basically I don't have a real response, this is just a gratuitous post in order to get to 3000 posts and receive the free CN T-shirt that everyone gets for reaching the milestone.

Edited by SteveC (12/15/13 10:41 PM)


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Roy McCoy
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Re: UPDATE: Prism Diagonals new [Re: SteveC]
      #6255437 - 12/15/13 11:00 PM

Woohoo!
Only fourteen more years and I'll get one too!


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JustaBoy
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Reged: 06/19/12

Re: UPDATE: Prism Diagonals new [Re: Roy McCoy]
      #6255462 - 12/15/13 11:23 PM

3000 CONGRATS STEVE!!!






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roadi
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Re: UPDATE: Prism Diagonals new [Re: JustaBoy]
      #6255644 - 12/16/13 03:35 AM

Quote:

I seem to remember reading in one of the old telescope making books, that for the most critical observations a Diagonal is never used...

But then men were a lot tougher back then, or at least in My Case, a Lot Younger! .




I have experienced this too many times, but do prefer relaxed viewing with diagonals and if so I found the least differences were between the prism/enhanced mirror vs straight through.


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BillP
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Re: UPDATE: Prism Diagonals new [Re: SteveC]
      #6255785 - 12/16/13 08:16 AM

I never got a T-Shirt at 3000, or any other number

And now that Steve is a part of the 300 ... I mean 3000, then his t-shirt should have the appropriate image -- Link.

Edited by BillP (12/16/13 09:18 AM)


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ThomasWos
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Re: UPDATE: Prism Diagonals new [Re: Don Taylor]
      #6259062 - 12/17/13 09:56 PM

Quote:

I'm looking to get a Baader T2-01C as well and understood that it does not have a Zeiss prism - but the short optical path solves some unique issues for my application.

BTW - it is also available from Agena (along with the other Baader diagonals)- although neither Apline nor Agena have any in stock at this time.




After waiting over a year, finally been able to order the T2 Baader prism diagonal from Telescope Service in Germany.

New Baader #BA2456005, I believe replaces T2-01C.

Arrives this week via DHL/USPS.


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etsleds
sage
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Reged: 11/14/09

Re: UPDATE: Prism Diagonals new [Re: ThomasWos]
      #6259147 - 12/17/13 10:40 PM

I've been pretty impressed with the large diagonal prism that's built into the Tak turrets, perhaps another one to add to the comparo?

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Don Taylor
sage


Reged: 07/12/09

Loc: South TEXAS
Re: UPDATE: Prism Diagonals new [Re: ThomasWos]
      #6259305 - 12/18/13 12:48 AM

Quote:

Quote:

I'm looking to get a Baader T2-01C as well and understood that it does not have a Zeiss prism - but the short optical path solves some unique issues for my application.

BTW - it is also available from Agena (along with the other Baader diagonals)- although neither Apline nor Agena have any in stock at this time.




After waiting over a year, finally been able to order the T2 Baader prism diagonal from Telescope Service in Germany.

New Baader #BA2456005, I believe replaces T2-01C.

Arrives this week via DHL/USPS.




Yes! This is the same part number shown on the agena site - also available with nosepiece and eyepiece holder as 2456005K

Agena says coming soon!

Alpine Astro told me they expected to receive these late November - but has not. I'm hoping soon. Otherwise I'll be ordering one from TS as well. Thanks!!!


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Don Taylor
sage


Reged: 07/12/09

Loc: South TEXAS
Re: UPDATE: Prism Diagonals new [Re: Don Taylor]
      #6262775 - 12/19/13 10:47 PM

Agena has both versions of the T2-01C instock - well, I guess I got the last 245006K.

Looking forward to trying in my smaller refractors.


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BillP
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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6298925 - 01/07/14 09:37 PM

FYI, just received the last two loaner diagonals, so will be doing a round of tests then publish a diagonal shootout article.

- Celestron 1.25" 1980s Prism

- Astro Tech 1.25" 99% Reflective 1/10th Wave Substrate Dielectric

- Takahashi 1.25" 100% Reflective Prism

- Tele Vue 1.25" Everbrite Dielectric 1/10th Wave

- Baader Zeiss 1.25" T-2 Prism

- Astro Tech 2" 99% Reflective 1/10th Wave Substrate Dielectric

- Baader 2" Prism

- Takahashi 2" Enhanced Aluminum Pyrex Mirror 1/10th Wave


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george tatsis
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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6299472 - 01/08/14 07:56 AM



George


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Fomalhaut
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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6299516 - 01/08/14 08:36 AM

Bill, since you're including two Baader prism diagonals in your testings I would personally have appreciated the inclusion of also one Baader (same brand!) Maxbright mirror diagonal with 140 EBC-layers. Or maybe also an AP Maxbright with 47 EBC-layers to see if there occurs any difference... Just because these two seem reputed to be the very best (AP in your country, Baader in Europe). And in order that nobody will be able to say that the best prisms topped the second best mirrors only...
Nevertheless appreciating your work.
Chris


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BillP
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Re: UPDATE: Prism Diagonals new [Re: Fomalhaut]
      #6299874 - 01/08/14 11:51 AM

Chris,
Unfortunately no one offered those to test. However, from all I have read most folks consider the TV Everbrite and the AP Maxbright to be equal optically. That being the case probably not much worth adding the AP to the mix. But don't know about the Baader one you mention. As a side, that many layers is kind of worrisome to me as more layers means more chances for imperfections and increasing impact from substrate imperfections as the layers build on that.


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fjs
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Reged: 03/25/13

Loc: Olympic Peninsula, USA
Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6300147 - 01/08/14 02:19 PM

Bill, If you want to use my AP Maxbright, PM me your address.

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Lance1234
sage
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Reged: 11/01/12

Loc: SoCal
Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6300204 - 01/08/14 02:45 PM

Quote:

Chris,
Unfortunately no one offered those to test. However, from all I have read most folks consider the TV Everbrite and the AP Maxbright to be equal optically. That being the case probably not much worth adding the AP to the mix. But don't know about the Baader one you mention. As a side, that many layers is kind of worrisome to me as more layers means more chances for imperfections and increasing impact from substrate imperfections as the layers build on that.




The Baader dialectric diagonal (at least the 2" Clicklock) is the only dialectric diagonal that I am aware of that specifically claims to maintain 1/10 wave accuracy after coating so it might be interesting to include. However, with the number of diagonals you already have, we'll look forward to hearing your results in a couple of years.


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BillP
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Re: UPDATE: Prism Diagonals new [Re: Lance1234]
      #6300238 - 01/08/14 03:10 PM

Quote:

However, with the number of diagonals you already have, we'll look forward to hearing your results in a couple of years.






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BillP
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Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6310458 - 01/13/14 03:01 PM

FYI, I am beginning a new thread on this in the Equipment Forum since this is not about eyepieces but about diagonals. Link. I have posted the pic of the final field of diagonal dreams 12 in total Including the infamous Vernonscope 1/20th wave quartz mirror!

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